2.A.68 The p-Aminobenzoyl-glutamate Transporter (AbgT) Family

The AbgT family consists of the AbgT (YdaH) protein of E. coli (Hussein et al., 1998) and the MtrF putative drug uptake transporter of Neisseria gonorrhoeae (Folster and Shafer, 2005). The former protein is apparently cryptic in wild-type cells, but when expressed on a high copy number plasmid, or when expressed at higher levels due to mutation, it allows uptake (Km = 123 nM) and subsequent utilization of p-aminobenzoyl-glutamate as a source of p-aminobenzoate for p-aminobenzoate auxotrophs (Carter et al., 2007). p-Aminobenzoate is a constituent of and a precursor for the biosynthesis of folic acid. MtrF is annotated as a putative drug efflux pump. However, it may catalyze drug uptake, allowing induction of another drug efflux pump (Folster and Shafer, 2005).

AbgT is 510 amino acyl residues long and has 12-13 putative transmembrane α-helical spanners (TMSs). MtrF is 522 aas long and has 11 or 12 putative TMSs. They are distant members of the Ion Transporter (IT) superfamily (Prakash et al., 2003; Rabus et al., 1999). Members of the AbgT family are found only in bacteria.

The abgT gene is preceded by two genes, abgA and abgB, which code for homologous amino acyl amino hydrolases and hydrolyze p-aminobenzoyl glutamate to p-aminobenzoate and glutamate (Carter et al,. 2007). Because of the structural similarity of p-aminobenzoyl-glutatmate to peptides, and the enzymatic activities of the abgA and abgB gene products, it is probable that AbgT is also a peptide transporter and glutamate (Carter et al., 2007). Demonstration of an energy requirement suggests an H⁺ symport mechanism (Carter et al., 2007). Expression of these genes is regulated by AbgR and an unknown effector.

The generalized transport reaction probably catalyzed by AbgT is:

p-aminobenzoyl-glutamate (out) + nH⁺ (out) → p-aminobenzoyl-glutamate (in) + nH⁺ (in).